Waste water entering a waste water treatment facility normally includes a substantial amount of large non-treatable material such as trash, garbage, cans, bottles, leaves, wood and plastic. If not removed at the outset, this material would collect in and interfere with the operation of the various treatment elements of the facility. A common approach to removing this material is to position a screen in the open channel through which the incoming water flows. The screen intercepts the material while allowing the water to continue its flow into the facility.
As the screen intercepts and retains the trash, the trash continually builds up on the screen. In order to prevent the trash from building up on the screen to the point where the flow of water is impeded, it is necessary to periodically remove the trash from the screen in some manner. The usual manner for removing the trash from the screen is to use a mechanical rake which rakes from the bottom of the screen toward the top. For example, one such device collects trash from the screen as it moves upward over the side of the screen presented to the incoming water, then moves over the top of the screen and out of the water flow and deposits the collected trash into a collector, then moves down the other side of the screen and under the bottom of the screen, and then moves back up the side of the screen presented to the incoming water to begin the process over again.
The rakes that are used to remove collected trash from the screen must be powered in some way. While it is possible to simply operate some type of rake assembly manually on an as-needed basis, the preferred approach is to power the rakes continuously with an electrical motor. The motor is used to drive a sprocket which propels a chain around the screen. The chain in turn is mounted to the rake assembly. See, for example, U.S. Pat. No. 3,591,00 by Daferner et al. Such motorized rakes are now very common in the industry due to their effectiveness, continuous operation and relative ease of maintenance.
A drawback to the use of motorized rake assemblies is that the motor may become flooded by the waste water. The motor is normally positioned such that it does not become submerged in the water. However, the water level in the incoming channel may vary quickly and dramatically due to abrupt changes in weather or due to high load conditions at certain times of the day. The rapidly rising water level can then partially or fully submerge the motor, thereby short-circuiting it or even destroying it.
One method for preventing the motor from becoming submerged in the incoming water is to include water sensors which detect the rising water level and then alter the operation of the rake assembly to move the motor above the water level. Such a method is disclosed in U.S. Pat. No. 4,857,182 by Jackson. A limitation to the sensor devices exemplified by the Jackson patent is that the motor may still become submerged if the water level is rising fast enough or if the water level rises at a time when the rake assembly is near the bottom of its travel under the screen.
Another approach to prevent the motor from becoming inoperative as a result of being submerged in the incoming flow is to enclose the motor in a waterproof housing. Thus the housing enclosing the motor may become submerged in the incoming flow of waste water while the motor itself remains dry. Such an apparatus is disclosed in U.S. Pat. No. 5,087,846 by Wright in which the enclosure comprises a two piece hermetically sealed housing. Aside from the fact that the apparatus described in the Wright patent interferes with normal air circulation to cool the motor, a limitation to the apparatus is that the housing may leak. The leakage is especially likely due to the temperature-induced pressure changes in the housing. For example, if the housing is sealed tight on a day when the ambient air temperature is 40.degree. F., and the full load operating temperature of the motor reaches 184.degree. F., then the interior air pressure will build to several pounds per square inch. Unless the housing seals are very good, some of the interior air will leak out. When the motor is then shut off and the interior air cools, there will be a negative interior air pressure. This negative interior air pressure combined with the positive exterior hydraulic pressure when the housing is submerged in the incoming flow, will force water through the seals and into the housing. It is possible to seal the housing with a positive interior pressure or to take care that the housing is sealed when the ambient air is warm, as on a hot day, but both of these approaches merely delay the problem of developing a negative interior air pressure leading to leakage, rather than prevent the problem.
Yet another approach to prevent the motor from becoming inoperative as a result of being submerged is to enclose the motor in an enclosure connected to a hollow arm of the screen, which when immersed in water pressurizes the enclosure. As the arm emerges from the water, the water in the arm drains out and the pressure returns to ambient. The enclosure is thus somewhat analogous to a diving bell, having an enclosed top to trap air and an open bottom. In such an enclosure the volume of air in the immersed arm of the screen must be exactly sized, or the enclosure is likely to "sip" water when a warm motor cools while the enclosure is submerged.